NO166051B - PROCEDURE FOR DETERMINING THE RESULTS SEAL IN A SUBSTRATE RESERVE. - Google Patents

Abstract

The residual oil saturation is determined by injecting water containing dissolved halocarboxylic acid salt and carbonate salt, into an oily and aqueous reservoir and chromatographic analysis of the relationship between the concentrations of the in situ formed CC> and of water-soluble acid-salt in fluid recovered from the reservoir.

Description

The present invention relates to a method

for the determination of residual oil saturation, in which a reactant-containing aqueous solution is injected into an underground reservoir to form tracers with different distribution coefficients for mobile and stationary phases of the fluid in the reservoir,

and measurements are made of the amount in which the tracers are separated chromatographically to determine the relative concentrations of these phases. The method is characterized by the fact that, as the aforementioned tracer-forming solution, an aqueous liquid is injected which, before or after introduction into the reservoir, contains dissolved salts of at least one halocarboxylic acid in contact with at least one carboxylic acid salt of types and in amounts suitable for reaction in the reservoir for formation of CC^ and salts of water-soluble acids in such amounts that measurable amounts of CC>2 are distributed between mobile and stationary phases of the reservoir fluids and anions of said acids dissolve essentially completely in the mobile phase of the reservoir fluid.

It appears that in conventional test operations the only tracer-forming reactants that have proved satisfactory have been hydrolyzable lower alkyl carboxylic acid esters such as those described in US Patent No. 3,623,842, or the analogous beta-ketoalkyl carboxylic acid esters, which are capable of to provide an unreacted ester or an unreacted ketone as the tracer that partitions between the water and the oil (or other mobile and stationary phases) and a tracer such as an alcohol (or an inert material) that dissolves practically completely in the water phase. Such previously known processes have found widespread use as a "single well tracer method", and more than a hundred jobs have been carried out. Generally, however, the reliance on organic esters has limited the use of the method to reservoirs with relatively low temperatures. Comparison of the track-forming abilities

(1) Temperature range

In typical previously known methods, an organic ester which is partially soluble in oil serves as the oil phase tracer which is injected at the wellbore and displaced to the desired distance from the wellbore by means of an inert fluid. A residence period then provides the necessary time for a hydrolysis reaction to take place and the formation of a significant amount of alcohol. The alcohol is not soluble in the oil and thus serves as a water phase tracer.

The hydrolysis step must not be too fast, as it is undesirable for the alcohol to form during the placement step, and there must also be a certain amount of unreacted ester left after the residence period, as this is the oil phase tracer. After the end of the residence period, both tracers are driven back to the wellbore. The degree of chromatographic separation of the two tracers is measured and used to calculate the residual oil saturation.

If the reservoir temperature is higher than 93°C, the rate of hydrolysis for most, if not all, known esters becomes so great that the above-mentioned requirements are not met. Therefore, the previously known methods have been limited to reservoirs of temperature. 93°C or less.

By means of the present method, this limitation is eliminated through the use of other tracers and other means for in-situ formation. In the present method, the oil phase tracer is CO-, . It is formed in situ during the residence period. The concentration of acidic anion does not change during the above reactions and serves as a tracer for the water phase. When the reactants include sodium chloroacetate and sodium bicarbonate, the reactions can be summarized as follows:

ClCH2COONa + H20 —► HOCH2COOH + NaCl

HOCH 2 COOH + NaHCO 3 fHOC^COONa + CC> 2 + H 2 O

A very large number of options have been available with respect to the choice of halocarboxylic acid. Examples of such acids are given in Table I, along with a best judgment estimate of the applicable temperature range for each of the CO2 generators listed: (2) Deeper penetration (investigation depth) from the wellbore

In typical known methods, the oil phase tracer is

an ester (ethyl acetate), which is injected into a carrier fluid. This ester is distributed between the oil and the water in the carrier fluid. The effect is a delay in the advance of the ester front

into the reservoir. In most cases, the ester will reach a distance corresponding to a volume of only approx. one third of the total volume of injected fluid.

By means of the present method, this problem is avoided, because the oil phase tracer (CG^ ) is not present in the fluids being pumped (except in unavoidable small amounts). The CC^ component is mainly formed after the placement is completed. Consequently, CG^ is formed at the leading edge and throughout the solution that is injected, and consequently the carbon dioxide is approx. three times (on a volume basis) further away from the wellbore than the ester system when the production part of the cycle starts.

Thus, according to the present system, the CC^-oil phase tracer will penetrate further into the formation than an ester system tracer (for a given treatment volume), and this will give a residual oil measurement and a volume that is approx. three times the reservoir volume that could be examined using the previously used system.

(3) Distribution coefficient

The distribution coefficient, Ki (the ratio between the concentration of the tracer in the oil phase and the concentration of the tracer in the water phase) for esters is approx. 6 in most cases. Ki for CC^ is approx. 2.

The Ki value for CO^ is in most cases very high

more optimal from a test sensitivity point of view, as more CC^ is present in the water phase, which constitutes practically all of the extracted fluid.

Also, CC"2 tracer f et will be returned to the wellbore much faster than an equivalent ester tracer.

If this characteristic is combined with the smaller volumes required for testing the reservoir due to the CC^ tracer's ability to penetrate deeper, it may be sufficient to do only minor jobs with CC^ tracer. In this case, several small CC^ tests could be carried out in different wells, instead of the one large ester test that is currently carried out. This will give better total values for the residual oil saturation (Ros = Residual oil saturation) in the reservoir than those that can currently be achieved.

(4) Operation during the penetration period

In most reservoirs, fluid that is injected into

a well, get a drift along with the total reservoir fluids when the pumps are stopped. It can be as much as one meter per day.

When using the ester system, long penetration periods are often necessary. This makes the drift a significant source of error, for which corrections must be made. Moreover, the method becomes less accurate and less sensitive.

With the present system, the large selection of acid generators that react at different rates at different temperatures, combined with faster backflow, will greatly reduce the effect of the drift in many cases. This is because the acid generators can be selected so that they respond more optimally to the reservoir temperature involved. Likewise, the water tracer and the oil tracer will stay very close to each other in the reservoir and consequently eliminate many of the errors introduced as a result of the drift velocity in the reservoir.

(5) Hydrolysis rate depending on the pH value

For most substances, the rate of hydrolysis is affected by the pH value. Esters have a constant rate that varies by about a factor of 10 for each unit change in pH (ie, the rate is proportional to the OH ion concentration). This proportionality means that the rate of hydrolysis for an ester decreases as the pH value decreases, as a result of the acid that is thereby formed. This means that there may be difficulties in getting enough alcohol to form in the reservoir so that the water tracer will become available for analysis.

( 6 ) Consumption of CO-,- generators

As it is the concentration of the anions of the CC^-forming molecules that is used as a water phase tracer, their hydrolysis and resulting release of CC^ involves no change in the number of anions. Thus, the concentration of the water tracer remains constant, regardless of the rate or degree of the hydrolysis reaction.

(7) Stripping of light hydrocarbons from the oil phase

The injection of brine from surface locations results in the dissolution of light hydrocarbons, such as methane and ethane, from the residual oil first encountered in the reservoir.

With the previously used unreacted ester tracer system, this "stripped" oil is the oil that is "stationary" ("immobile") during the chromatic production operation. During the injection, the injected water front moves much faster than the ester front, and it is unlikely (1) that the light ends which are thus "stripped" from the oil, will be brought back together with the oil when the water and ester are produced back to the wellbore and (2) that this will happen before the ester comes into contact with the oil again.

In the present CC>2 system, a corresponding stripping will take place during the injection. However, the production cycle is such that a significant amount of local reservoir water will have flowed past any stripped oil before any CC>2 has come into contact with the stripped oil. Naturally, the carbon dioxide has not previously been in contact with this oil, in contrast to what is the case in the ester system.

(8) Miscellaneous

(a) A more accurate location of the carbon dioxide in the reservoir may make it possible to use frontal analysis techniques on the tracers instead of the band analyzes used for the ester. Frontal analyzes should be more accurate. (b) In some cases very small amounts of C<>> can

be sufficient as a result of the great sensitivity and stability of the analysis systems.

(c) If the drift is minimal, it may be possible

to use simple methods to analyze the data and to calculate the residual oil saturation.

Generally, the present method will be able to be used in all the reservoir situations and with all the methods for determining fluid saturation where the previously known methods were suitable.

The halogen-organic acid salt used in the present process can be virtually any salt that is water soluble, hydrolyzes and reacts with carbonate salts to form CC₂ and a water soluble salt, and is compatible with the fluids and the solids in the reservoir and the other components of the tracer-forming solution to be injected.

The carboxylic acid salt which is suitable for use in

the present method, can be any water-soluble carbonate or bicarbonate containing cations which can form water-soluble salts with the hydrolysis product of the hemicarboxylic acid salt and are compatible with the reservoir materials and other constituents of the injected tracer-forming solution. Also solid carbonate salts or dolomitic salts (such as CaC^ , FeCO^, MgCO^^ which may be present in the reservoir are suitable.

A tracer-forming solution suitable for use in the present method comprises an acid-forming agent consisting essentially of at least one halocarboxylic acid salt dissolved in an aqueous solution that contains or will come into contact with salt when the solution is injected into the reservoir to be treated, and at least one acidic, reactive carbonate or bicarbonate. The tracer-forming solution preferably contains a sufficient amount of substantially neutral salt and pH-adjusting acidic or basic material to provide a composition that is at least compatible with, if not substantially equal to, the aqueous liquid present in the reservoir to be are tested. The combination of the types and amounts of halocarboxylic acid and carboxylic acid salts is preferably adjusted according to the reservoir temperature, so that easily detectable amounts of C0 ? and hydroxycarboxylic acid anions in the respective mobile and stationary liquid phases in the reservoir. What is important for this is that the trace-forming solution, at least soon after it is introduced into the reservoir, contains a sufficient amount of carboxylic acid salt to form a sufficient amount from the acid formed by hydrolysis of the halocarboxylic acid anions. When the reservoir contains water-soluble or -insoluble carbonate constituents, the tracer-forming solution, as injected, may be substantially or even completely free of carbonic acid salt, until this solution comes into contact with the reservoir formation and the carbonic acid salt dissolves from and/or diffuses from the reservoir the rocks or reservoir fluid into the tracer-forming solution.

Table II sets forth results obtained by testing various CC₂ generators at various temperatures and pH values. In each case, the solution was held at a pressure of 344.7 kPa during the test. The pH value of the solution was kept practically constant by adding portions of 0.1 mol/l sodium bicarbonate solution to the system during the course of the hydrolysis. Each acid generator solution consisted of water containing 0.5 mol/l sodium chloride and 0.05 mol/l of the specified halocarboxylic acid salt.

The patterns of the concentrations as a function of the amounts of fluid extracted from the reservoir being tested (and/or the concentrations as a function of time when the production rate is essentially constant) can be measured using commonly known and available methods and apparatus. It is a clear advantage of the present method that known and available, relatively simple procedures, such as titrometric and/or thermometric analyses, can be used to measure the chromatographic separation between the CC>2 that is distributed between the phases and the acid anions that are practically dissolved counted completely in the mobile phase of the reservoir fluid.

In the present system, the pH value is far less important for regulating the hydrolysis rate, as this changes by only approximately a factor of 3 per unit change in pH (see Table II, Tests 1 and 2). Also, the reaction proceeds more rapidly as the pH falls (ie, the rate is proportional to the number of H ions). This means that the rate of hydrolysis of the CC^-formers will increase as the acid is formed and give a much more reliable amount of CC^

from test to test. Furthermore, if the CO 2 -former is completely depleted in the reservoir, no difficulty will arise, because the water tracer depends on the acidic anion associated with the CC 2 -acid-former.

Claims (5)

1. Procedure for determining residual oil saturation, in which a reactant-containing aqueous solution is injected into an underground reservoir to form tracers with different distribution coefficients for mobile and stationary phases of the fluid in the reservoir, and measurements are made of the amount in which the tracers are separated chromatographically to determine the relative concentrations of these phases, characterized in that, as said tracer-forming solution, an aqueous liquid is injected which, before or after introduction into the reservoir, contains dissolved salts of at least one halocarboxylic acid in contact with at least one carboxylic acid salt of types and in amounts suitable for reaction in the reservoir to form CC ^ and salts of water-soluble acids in such amounts that measurable amounts of CC>2 are distributed between mobile and stationary phases of the reservoir fluids and anions of said acids dissolve essentially completely in the mobile phase of the reservoir fluid. 2. Method according to claim 1, characterized in that oil and water are used as the mentioned phases. 3. Method according to claim 1 or 2, characterized in that the pH value for the injected fluid is set to approximately the pH value for the aqueous fluid in the reservoir being tested. 4. Method according to claims 1-3, characterized in that the types and quantities of the tracer-forming fluids are adjusted so that the quantities of the tracers are provided during a selected, relatively short time. 5. Method according to claims 1-4, characterized in that the injected fluids are extracted by taking them out through the well through which they were injected.